1.1 Paste Chemistry & Alloy Choice

Flux systems, powder sizes, SAC vs SnPb—and how each choice widens (or shrinks) your window of process.

Solder paste is a mix of alloy, flux, and powder size—and each choice directly affects printing, reflow, and final reliability. Flux can be no-clean (long stencil life, benign residue) or water-soluble (aggressive wetting but requires washing). Powder size ranges from coarse (Type 3, forgiving but limited for fine pitch) to fine (Type 4/5, better for small apertures but shorter print life and more oxidation risk). Alloy sets melting temperature and mechanical behavior—SnPb runs cooler and wets easily but isn’t RoHS; SAC305 is the lead-free workhorse with a higher peak and broader “mushy” zone; Bi-based alloys allow low-temp reflow but have mechanical tradeoffs. Match powder to stencil apertures, flux to your cleaning capability, and alloy to your reliability and temperature limits—then prove the choice with SPI, AOI, and AXI data under a tuned reflow profile.

1.1.1 What’s in a solder paste (and why you should care)

A paste is alloy + flux + powder size. Alloy sets melting temp and long-term behavior; flux decides print life, wetting, residue/cleaning; powder size unlocks your fine apertures without slumping or clogging. Every knob you twist here shows up in printing (7.2–7.4) and reflow (8.x) later—so pick on purpose.

1.1.2 Flux systems (no-clean vs water-soluble)

Flux family	What it’s like on the line	Where it shines	What to watch
No-clean	Long stencil life, forgiving of small pauses; leaves benign residues when process is in control.	Most consumer/industrial builds; no wash processes.	Cosmetics and residue rules—confirm with customer; borderline wetting may benefit from N₂. (See 9.3 & 15.1.)
Water-soluble	Strong activators for fast wetting; requires a wash (and a real cleaning process).	Dense BGAs/QFNs where you want aggressive flux then remove it.	Adds cleaning station, fixtures, chemistry, and verification (Chapter 15).

If in doubt, start no-clean, prove residues are acceptable, and keep water-soluble in your back pocket for stubborn wetting/voiding cases. (We’ll cover cleaning choices in 15.1–15.2.)

1.1.3 Powder size (why Type 3/4/5 isn’t just marketing)

Smaller powder (e.g., Type 4/5) prints tighter apertures but oxidizes faster, narrows print/hold life, and can demand N₂ to keep wetting crisp. Larger powder (Type 3) is forgiving but taps out on very fine pitch. Match size to your aperture area/aspect ratios and component mix (we’ll do the math in 7.4).

Fast rules

If your area ratios are marginal, move up a powder class or step the stencil locally (7.3/7.4).
Tighter powder → tighten storage/handling (7.2) and consider N₂ for reflow (9.3).

1.1.4 Alloy choice (what changes in reflow and life)

Alloy family	Melt behavior (vs others)	Process vibe in reflow	Reliability notes
Sn63/Pb37 (eutectic)	Lowest peak of the common choices; sharp liquidus	Wide thermal margin; easy wetting	Not RoHS. Different intermetallic growth vs SAC; profiles differ (see 9.4).
SAC305/lead-free SAC	Higher peak than SnPb; broader “mushy” zone	Needs honest profiling (TAL/peak matter); N₂ often helps cosmetics/voids	Widely used; doped SAC & specialty mixes exist for harsh duty. See 9.4 & 16.2.
Low-temp (Bi-based)	Much lower peak	Great for heat-sensitive parts/boards	Narrower process window; mechanical limits—qualify carefully. (We’ll profile it in 9.2/9.4.)

Different alloys want different profiles—don’t reuse SnPb curves for SAC or Bi mixes. We’ll build data-driven curves in 9.2 and discuss alloy nuances in 9.4.

1.1.5 Map paste choices to your process window

Think window in three axes: print, reflow, cleanliness.

Print axis: Powder size + rheology must hold volume/height/area steady across the shift. If you’re living at the edge, change powder/stencil—not operator heroics. (SPI & closed loop in 7.6.)
Reflow axis: Alloy + flux decide TAL, peak, and need for N₂. SAC and low-temp pastes often benefit from nitrogen when pushing cosmetics/voiding. (9.2–9.3.)
Cleanliness axis: No-clean saves time—if residues meet spec; water-soluble widens wetting but adds a whole cleaning process to control. (15.1–15.2.)

When windows feel narrow, run a small DOE: two powders × two atmospheres × two peak temps. Keep what moves your SPI→AXI Pareto the right way. (Window finding in 16.4.)

1.1.6 Starter picks (use, then tune with data)

General lead-free builds: No-clean SAC with Type 3–4; air reflow unless BGAs/voiding push you to N₂.
Fine-pitch BGA/QFN: No-clean SAC, Type 4 (or 5 if area ratios demand), consider N₂; pair with windowed apertures on thermal pads. (See 7.4.)
Heat-sensitive boards/parts: Low-temp Bi-based paste; validate drop/mech and tune gentle profiles. (9.2/9.4)
Legacy/service builds: SnPb where allowed; profile separately from SAC. (9.4)

1.1.7 Don’t forget the care & feeding

Great paste poorly handled is bad paste. Control storage, thawing, mixing, open time—we’ll set those rules in 6.2 and tie them to shelf life in Materials (17.4).

1.1.8 Release checklist (print this before you buy)

Flux family chosen (no-clean vs water-soluble) and cleaning plan aligned.
Powder size matched to smallest area/aspect ratios in your stencil set. (See 7.4.)
Alloy picked with reflow profile plan (TAL/peak; air vs N₂). (9.2–9.4)
Handling & shelf-life rules documented (storage, thaw/mix, open time). (7.2, 5.5)
DOE plan ready if the early lots feel tight; tie reads to SPI/AOI/AXI dashboards. (7.6, 10.x, 16.4)

Bottom line: pick a flux you can live with, a powder your apertures can print, and an alloy your boards and customers can tolerate—then prove the window with real SPI→AXI data and a profile that fits the chemistry. Lock it in, and printing gets calm, reflow gets boring, and yields get happy.